1. Field of the Invention
The present invention relates to thermoplastic resin composition which are low in molding shrinkage and in thermal deformation, have excellent surface characteristic, high strength and low deformation and is suitable as molding materials for plastic molded articles which have complicated shapes and must have high accuracy, e.g., components of electronic appliances, such as pickup parts of a compact disk player and ferrules of optical fibers.
2. Description of Related Art
A group of plastics, the so-called engineering plastics, are replacing metallic parts by virtue of their high strength. However, most of the plastics called engineering plastics have a molecular structure known as crystalline polymeric structure, so that they have a disadvantage of large so-called molding shrinkage. It is a present situation that, in a practical use, the disadvantage is somewhat compensated for by appropriately setting molding conditions, particularly the design of a mold.
In general, in order for a polymer to be a material having high strength, it is desirable that the polymer be a crystalline polymer having ordered molecular arrangement. Since, however, changes in the state of such a polymer from a molten state to a solid state means changes in its form from an amorphous form to a crystalline form, it is impossible to solve a fundamental problem that the volume change of the polymer is inevitably larger than that of a noncrystalline polymer. The fact that the balance between deformation and properties is particularly important in this case makes it difficult to solve the problem.
The currently used materials will now be reviewed from this point of view. An unfilled resin exhibits relatively large molding shrinkage and small stiffness. On the other hand, a composition containing a particulate material is small in molding shrinkage, and is low in strength as well. Further, a composition containing a fibrous material is high in both strength and stiffness, but tends to exhibit a large molding shrinkage. Therefore, it is quite difficult to improve the stiffness and strength without causing an increase in the molding shrinkage. Particularly, it is a current situation that no satisfactory compositions are found with regard to crystalline resins.
However, in recent years, the development of a thermotropic liquid crystal polyester which exhibits anisotropy in a molten state changed the whole situation. Since this liquid crystal polyester melts while maintaining the crystalline structure, the resulting moldings advantageously have a combination of high strength derived from its crystalline structure with a small difference in the volume between a molten state and a solid state, i.e., a small molding shrinkage, attributable to the fact that the crystalline structure does not significantly change when it is solidified. However, this also has a drawback. Namely, although the absolute value of the molding shrinkage factor is small, the difference in the molding shrinkage factor between the direction of resin flow and a rectangular direction during molding, i.e., the anisotropy in the molding shrinkage factor, is large, which makes it difficult to obtain precision moldings.